Also, bear in mind that this is right before we run the memory allocator. Shaving a few cycles off here is kind of like chipping a pebble off of an asteroid.
On Mon, Nov 30, 2015 at 10:37 PM, Dan Cross <[email protected]> wrote: > Numbers of it didn't happen. > > On Mon, Nov 30, 2015 at 9:49 PM, 'Davide Libenzi' via Akaros < > [email protected]> wrote: > >> The MUL_NO_OVERFLOW thing makes little sense in the CPUs we are targeting. >> A couple of mis-predicted branches are more expensive than a DIV (~20 >> cycles in today Intel). >> Much faster: >> >> typedef unsigned __int128 uint128_t; >> >> { >> uint128_t n = (uint128_t) nmemb * (uint128_t) size; >> >> if (n > SIZE_MAX) foo(); >> >> } >> >> >> -------------------- >> typedef unsigned __int128 u128; >> >> static const unsigned long ulmax = ~0UL; >> >> unsigned long oc(unsigned long a, unsigned long b) >> { >> u128 m = (u128) a * (u128) b; >> >> if (m > ulmax) >> m = 0; >> >> return (unsigned long) m; >> } >> >> ---- >> oc: >> movq %rdi, %rax >> mulq %rsi >> movq %rax, %r9 >> xorl %eax, %eax >> testq %rdx, %rdx >> cmove %r9, %rax >> ret >> >> >> >> >> >> >> >> On Mon, Nov 30, 2015 at 1:18 PM, Barret Rhoden <[email protected]> >> wrote: >> >>> One thing to be careful of is that these functions can fail and return 0, >>> regardless of your kmalloc flags. >>> >>> Signed-off-by: Barret Rhoden <[email protected]> >>> --- >>> kern/include/kmalloc.h | 5 +++-- >>> kern/src/kreallocarray.c | 9 +++++++++ >>> 2 files changed, 12 insertions(+), 2 deletions(-) >>> >>> diff --git a/kern/include/kmalloc.h b/kern/include/kmalloc.h >>> index 268cf2985ddb..26f23c1754c0 100644 >>> --- a/kern/include/kmalloc.h >>> +++ b/kern/include/kmalloc.h >>> @@ -16,8 +16,9 @@ >>> #define KMALLOC_LARGEST KMALLOC_SMALLEST << NUM_KMALLOC_CACHES >>> >>> void kmalloc_init(void); >>> -void* kmalloc(size_t size, int flags); >>> -void* kzmalloc(size_t size, int flags); >>> +void *kmalloc(size_t size, int flags); >>> +void *kmalloc_array(size_t nmemb, size_t size, int flags); >>> +void *kzmalloc(size_t size, int flags); >>> void *kmalloc_align(size_t size, int flags, size_t align); >>> void *kzmalloc_align(size_t size, int flags, size_t align); >>> void *krealloc(void *buf, size_t size, int flags); >>> diff --git a/kern/src/kreallocarray.c b/kern/src/kreallocarray.c >>> index d099721e0406..505159917317 100644 >>> --- a/kern/src/kreallocarray.c >>> +++ b/kern/src/kreallocarray.c >>> @@ -34,3 +34,12 @@ void *kreallocarray(void *optr, size_t nmemb, size_t >>> size, int flags) >>> } >>> return krealloc(optr, size * nmemb, flags); >>> } >>> + >>> +void *kmalloc_array(size_t nmemb, size_t size, int flags) >>> +{ >>> + if (((nmemb >= MUL_NO_OVERFLOW) || (size >= MUL_NO_OVERFLOW)) && >>> + (nmemb > 0) && ((SIZE_MAX / nmemb) < size)) { >>> + return NULL; >>> + } >>> + return kmalloc(size * nmemb, flags); >>> +} >>> -- >>> 2.6.0.rc2.230.g3dd15c0 >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "Akaros" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to [email protected]. >>> To post to this group, send email to [email protected]. >>> For more options, visit https://groups.google.com/d/optout. >>> >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Akaros" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To post to this group, send email to [email protected]. >> For more options, visit https://groups.google.com/d/optout. >> > > -- You received this message because you are subscribed to the Google Groups "Akaros" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. For more options, visit https://groups.google.com/d/optout.
